romandian

so i took the 07c6z to the track first time. i got it with brakes, polys, drms, radiator (and seat + harness). 265/35/18, 325/30/19 cup2s (vintage17), 1/8" of thead. 64f° outside. tires stabilised ar around 105 f° 33 psi. even across the thread.

the car came with a 31 mm front sway that, i replaced with t1 (38 mm). rear was 29 mm sway (no idea if hollow). i was chichen i would understeer.

well, after 3-4 rounds the the car strated oversteering, the tail got twitchi during braking and i couldnt run down the rerraris any more. i got 3 rounnds of this, going into the pits after 6-8 rounds (measuring tire temp.).

so, whats wrong, a massive front bar. tires, they might be cycled out. but why one axle diff. than the other?

the "front overbared" car oversteers?

i was hoping to whipe them all out by giving full throtle before apex, but not so.

any ideas to whats going on?

i was going to order a 315/ 335 combo. having second thoughts. also, how do the sqare guy make out? no idea.

whats my Problem?

edit: 2.7/1.7 camber, nearly zero toe in f and r. (< 0.1°).

Walt G

I'm far from an expert on this, but loose under braking could be too much rear brake. Since this came on after a few runs, maybe some pads or rotors got overheated and aren't working the same anymore (more bite on the rear, or less on the front).

My C5Z had an on-power oversteer my first track day (running square tire setup, stock bars) and a little rear toe-in really helped... that's an easy (and free) thing to try.

Hope that helps.

firebirdfan

Not enough rear toe. Add more.

davidfarmer

romandian

stoptech 355mm in front (semi race pads, they stop cold, supposedly cobalt (?), didnt even know they existed), stock rears with same pads. as any other vette car overbrakes in front. cant say more about temps, was measured three times with a needle type (stick in 2 mm) pyrometer in the pits, after a cool off round though. was stable at 41°c front and 40°c rear, guessing error at +/- 1°. a bit weird, probably cool off being the cause, as at 25°c ambient they get to 40° also on the freeway (80 mph). but the point is they were the same accross the thread and the same on all corners, so not overinflated. (it was the first outing after 15 years, but i was not driving too timidly, as i was able to follow the ferraris (i got one on the straight) and 3 c7s (one must have been superchaged, was getting away on the straight).

i took out 0.5° of toe in in the rear after getting the car, thinking it was stupid, because everybody used to run zero toe on track in my former life. i had a lot of research in this, talking to many racing teams. so how has the world changed? how (why) will rear toe in help, because its not only about exit but also before apex without throttle. 38 mm front bar and 29 mm rear (middle hole) oversteering with a smaller front tire that stock. still dont get it.

edit; track is small, about 3 miles, but gets you to 160 mph if you want/can (i dont want). anneau du rhin: http://www.rheinring.com/rennstrecke...trecke-4-0-km/

tw78911sc

Without rear toe, when the rear suspension is under load the outside tire will actually end up toe'd out causing rear steer.

romandian

o.k., so how much rear toe in is needed? because there is a trade off to steering in, right?

and i still dont get the picture. a change of 0.5° of rear toe, how would that cause the car to breake loose under cornering load? i always thought it was all about weigt transfer, gripp and power being applied.

sccaGT1racer

The car needs to be gone through and everything checked. I would start with checking the suspension for binding. Poly bushings are great for this. You should add some rear toe in but not .5. The tires may be cycled out but with your setup and tire sizes it should be pushing.

I haven't checked a c6z yet but a c5z that I checked had almost a half an inch of bump steer.

romandian

not sure what your saying. bump steer? in the rear? suspension is stock everything except polys and bars. what could be binding? stock wheels, tires stock rears smaller fronts.

CaddyRacer

check your front bushings.
I had rear end twitch under heavy braking. I was told alignment, brakes, tires, bump steer and "that's normal"
I found out the one front a arms bushing looked bad so they were all replaced under GM warranty that twitch is gone

Dropspeed

Start with the absolute basics before you start changing things.

You stated this started after 3/ 4 rounds. Typically this means something broke or your tires are heating up and getting greasy.

Was the 33psi cold or hot. Was this consistent all day? Typically the front tire pressure will increase more than the rears, but that is track dependent.

When I track I always leave the dash on the front pressure so I can see when they come up to temp and when they will start falling off. Maybe try this with the rear so you can see what the pressures increase to and see what the car is doing / how does it feel.

-Matt

l98tpi

paul69camaro

Romandian,

I like to run .15 degrees of toe in per side so .3 total rear toe in. As someone mentioned earlier, as the rear suspension on these cars compress, the wheels begin to toe out. This is made worse if the car is lowered without adding a rear bumpsteer tierod setup such as the one Baer offers.

cwikstro

I might suggest that you go through the process from a new to me approach and assume nothing. Take it to a good alignment shop and make sure all 4 corners are correct. You can set a good basic track or track/ street set up there (available online if you don't have this). This will be a good time to have them or you check all suspension parts. You don't say age, miles, etc, but assume nothing and go through all of it. It is common for little things to effect the handling, such as a loose ball joint or similar slop. I would get a new set of tires with known status, and also do the same for brakes. Unless your baseline is with new and known parts, you really can't chase these problems.
With all set and known, then do your sessions and take readings, notes etc until you get what you want. Also only change one thing at a time and then test again. Good luck!

Nowanker

Especially the tires. Can't be sure of anything with questionable rubber.

romandian

thanks folks.

so ill try 0.3° of rear toe and get new tires anyway for next year. i know they are done, but why more in the rear? i have polys about 2 years old (5k miles?) that look perfect and 33 psi was when comming into the pits. nothing broke because it was the same each time after a break.

my dilema is that i was going to switch from the current 265s to 315s in front (which i understand is a standart setup) and it seems a really bad idea now.

The HACK

This is by far the most logical answer, whether it's correct or not.

You can't solve this problem by simply throwing changes to the car first. You have to understand WHY it's oversteering at corner entry. Without knowing why, whatever you do to it will only likely be a bandaid to the gun shot wound.

romandian

mystery solved. po had glued aluminum shims to the mounting bosses of the brackets. they were rectangular and invisible. i beat them off and now cars understeers badly. i cant move the bar by hand anymore however, so much to the idea it has to be free moving. if you shim that bar so it can be moved you loose hals of its rate.

Nowanker

Can't really get on board with that fix. Introducing friction into suspension movement might be likely to cause erratic handling. Sudden snap oversteer, when that F bar finally gets loaded enough to overcome the resistance?
I still vote for aged-out tires, but I've also had bad experiences with failing shocks.